The invention relates to a method for sputter-induced precipitation of metal oxide layers on substrates by means of a reactive sputter process and also to a layer system produced according to such a method.
Methods of manufacture of this kind and single-layer systems manufactured according to this process are known. By means of a sputter device as described for example in DE 4,106,770, substrates are coated by means of cathode sputtering, preferably by magnetron cathode sputtering, where so-called targets are exposed to the action of a plasma cloud which forms between two electrodes, and where the evaporated target material which shows affinity to the reactive gas, forms a chemical bond with the gas and precipitates upon the substrate. In the known sputter methods the target is for example the one electrode and the substrate the other electrode which is connected electrically to both outputs of an electric power supply device. As an alternative, so called double electrodes are also used, which electrodes are alternately switched as anode and cathode. Electric power is supplied to the electrodes either as direct current or as alternating current, as described for example in DE OS 3,802,852.
For precipitation of dielectric layers such as for example SiO2, Al2O3, ZrO2, TiO2, ZnO2, the target to be evaporated is composed of the corresponding metal components present in the above compounds and is acted upon by the plasma cloud composed for example of Ar/O2 or Ar/N2 mix, which action evaporates the metal target.
A problem long known in the use of the generic sputter methods has been to provide homogenous and uniformly composed layers of high optical quality reproducible for industrial use. Thus dielectric layers such as for example SiO2, Al2O3, or especially TiO2, deposited for example by the DC sputter method on glass surfaces or optical components, have shown less than the desired resistance to environmental conditions, such as for example humidity acting upon the layers. Sputter and coating techniques realized by means of the DC sputter method additionally cause in a disadvantageous manner long and therefore expensive coating times during which the process parameters defining the coating process must be held constant. In addition, the optical properties of layer systems produced using conventional sputter methods have proved inadequate increasingly exacting requirements.
An object of the present invention is to provide a method of sputter-induced production of metal oxide layers on substrates by means of a reactive sputter process through which metal oxide layers of high optical quality can be made available for industrial manufacture in a reproducible and cost-efficient manner.
According to the invention, the object is accomplished by a method of this kind as mentioned in the introduction according to claim 1 in that electric power is applied to the plasma charge acting upon the sputter target to be evaporated by means of at least two electrodes arranged in the vicinity of each other in the plasma reaction space, and where electric power is selected such that oxide layers to be precipitated on the substrate to be coated are deposited at a coating rate of  greater than 4 m/s, whereby during the coating process the substrate to be coated is arranged stationary in relation to the target material to be evaporated. A coating rate of  greater than 40 nm/min is proposed for substrates which during the coating process are to be moved in front of the sputter target as in so-called continuous systems. Metal oxide layers produced according to the method of the invention exhibit, surprisingly enough, several advantages vis-à-vis metal oxide layers produced by conventional sputtering. Thus it was found that TiO2 layers produced according to the invention had a refractive index n between 2.55 and 2.60. Conventional DC technique only produced n values between 2.35 and 2.45. Metal oxide layers having a high n value advantageously allow a thinner metal oxide layer than one produced by conventional methods in order to achieve an effect dependent on the refractory value. In addition, thinner metal oxide layers have the advantage of high light transmission and color neutrality in the visible spectrum. Moreover, thin metal oxide layers can be produced more cost-effectively than conventional metal oxide layers.
Layers produced according to the invention also advantageously exhibit a very smooth surface. Surface structure morphology of metal oxide layers produced according to the invention exhibits a very compact crystalline definition which demonstrates high resistance to chemically reactive substances. Metal oxide layers produced according to the invention are correspondingly more resistant to the effects of humidity than conventional sputter layers produced for example by means of a plasma discharge. Furthermore it was found that by using sputter plasma operated with alternating current the precipitated TiO2 layers crystallized primarily in a rutile structure. Contrary to the anatase structure of the TiO2 layer which primarily forms in DC sputter process, the rutile structure is temperature-stable up to 1855xc2x0 C., while the anatase structure undergoes a phase conversion at 642 xc2x0 C. and exhibits an unstable structure. It has been further shown that given equal plasma output, the process according to the invention achieves a sputter rate about 6 to 7 times higher than that of conventional DC sputter process.
Metal oxide layers produced according to the invention can also be used for improved low-E layer systems and for so-called solar-control systems with improved optical properties. In the case of low E layers the possibility is created advantageously to forgo tin as the target material in favor of economical titanium. Since SnO2 layers produced by the sputter process disadvantageously tend to develop islands, it is desirable to substitute TiO2 for SnO2. In the case of low-E layer systems the base layer applied directly on the substrate exhibits according to the invention a smooth and compact surface structure onto which the actual low E layer is applied, for example a silver or gold layer. Morphology of the base layer produced according to the invention also advantageously promotes the forming of an applied metal layer which exhibits high conductivity or a low k value, respectively.
It has been shown to be advantageous for the production of layers according to the invention to select a frequency of the alternating current supplying the sputter plasma between 10 kHz and 80 kHz.
Additional advantageous features of the method and possible uses of metal oxide layers according to the invention are characterized in the following description.